Here's my situation, I have a 150GB Raptor X for my main hard drive (almost full), I have a second drive, a 320GB WD I use for backup. Basically, I just clone my 150 to the 320 every week. I want to sell the raptor x and the 320 and get two 640GB WDs. Will there be any sort of performance gain/loss between a raptor 150 and a 640WD?
There will be a performance loss. Whether or not it is noticeable really depends on how you use the harddrives. The 640WD is a very good replacement. If it's more storage you are after, I wouldn't worry about the minor performance hit.
If you want performance, sell the raptorX and replace it with a velociraptor-300. You will get the capacity increase that you need, and a performance increase too. Keep the 320 drive for backup as you have been doing.
If you need more than 300gb of active data, the two 640 gb drives is an ok solution.
I think, though, I would keep the raptorX for the highly active data, use the 320 drive for overflow, and get a third 640gb drive for backup.
I agree with geofelt. 150 gigs is more than enough for your operating system and whatever programs/games you want to run quickly. Keep it for the speed, and buy cheaper larger drives for your massive amounts of data that you don't need the speed, like mp3s, etc.